This invention relates to cooled, variable geometry exhaust nozzles for turbine engines and particularly to a simple and effective arrangement for supplying coolant to the interior of the divergent section of a variable geometry, convergent divergent nozzle.
A typical gas turbine engine for high performance military aircraft often employs an afterburner and a convergent divergent (CD) exhaust nozzle. The nozzle circumscribes an axially extending engine centerline and radially bounds the aftmost portion the engine""s main gaspath. A stream of hot, gaseous products of combustion flows axially through the nozzle to produce thrust for powering the aircraft. The CD nozzle includes a convergent section, whose cross sectional area converges in the direction of gas flow, and a divergent section whose cross sectional area diverges in the direction of gas flow. The convergent and divergent sections are serially arranged to define a throat, which is the smallest cross sectional area of the nozzle. The aft extremity of the divergent section defines a nozzle discharge plane and a corresponding discharge area.
It is often necessary to cool the nozzle to protect it from the heat of the combustion gases. One effective cooling technique is film cooling. Film cooling involves flowing a film of coolant along the radially inner surface of the nozzle, which is the surface directly exposed to the gases. Unfortunately, the effectiveness of the coolant film progressively decays as it flows along the surface being cooled because the coolant intermixes with the hot combustion gases. Therefore, film cooling alone may not be satisfactory for nozzles that are particularly long or that are exposed to extremely high temperatures. In such nozzles, the film cooling is augmented by internal convective cooling in which coolant flows through an interior space in the nozzle.
Many CD nozzles are variable geometry nozzles. Variable geometry nozzles comprise a set of convergent flaps pivotably connected at their forward ends to an engine case and a set of divergent flaps pivotably joined to the aft ends of the convergent flaps. The flaps may be monolithic, or they may be constructed of radially inner and outer skins separated by spacers. The angular orientation of the flaps, and therefore the geometry of the nozzle, is governed by a control system that includes an automatic controller and associated actuators and mechanical linkages. During engine operation, the control system causes the convergent flaps to pivot relative to the case and the divergent flaps to pivot about their respective connections to the convergent flaps. By governing the orientations of the flaps, the control system adjusts the throat and nozzle discharge areas.
Variable geometry nozzles are superior to fixed geometry nozzles in that the adjustability of the throat and discharge areas allows for optimized performance over a wide range of operating conditions. However they also suffer from disadvantages relative to fixed geometry nozzles. One of these disadvantages is the difficulty of introducing convective coolant into the interior space between the skins of the divergent flaps to augment the film cooling applied to their gaspath exposed surfaces. According to past practice, each divergent flap includes inlet openings at its forward end and exhaust openings aft of the inlet openings. The inlet and exhaust openings cooperate with the interior space between the flap skins to define a convective coolant flowpath. During operation, the inlet openings capture some of the film coolant flowing along the radially inner surface of the nozzle. The captured coolant flows through the interior of the flap and discharges back into the combustion gas stream through the exhaust openings. Although this arrangement is satisfactory in some applications, its effectiveness may be limited for at least two reasons. First the inlet openings necessarily reside in the divergent flap, which is aft of the nozzle throat. Because the pressure of the combustion gases drops precipitously as the gases flow across the throat, the pressure gradient from the inlet openings to the exhaust openings may be too small to encourage adequate convective coolant flow through the convective coolant flowpath. Second, the openings themselves reside in a plane substantially parallel to the direction of flow of the film coolant, and therefore are poorly oriented for capturing part of the coolant film.
One possible way to achieve more effective convective cooling is to capture a portion of the film coolant at a location forward of the throat where the combustion gas pressure is relatively high, and channel the captured coolant across the joint between the convergent and divergent flaps. However this approach requires that a leak resistant coolant path be established to bridge across the joint. Such a coolant path must accommodate the relative motion between the convergent and divergent flaps, and therefore introduces undesirable weight, cost and complexity.
What is needed is a simplified arrangement for effectively introducing coolant into the interior of exhaust nozzle divergent flaps.
It is, therefore, an object of the invention to effectively cool the divergent flaps of a convergent divergent exhaust nozzle and to do so with a minimum of weight, cost and complexity. It is a further object of the invention to provide a simple way to introduce convective coolant into the interior of the divergent flaps.
According to the invention, A variable geometry exhaust nozzle includes convergent and divergent flaps that circumscribe a gaspath and define convergent and divergent nozzle sections with a throat therebetween. A projection, which ideally resides on the divergent flaps, extends radially inwardly from the nozzle forward of the throat. The nozzle also includes a coolant flowpath comprising an interior space in the nozzle, an intake for admitting coolant into the interior space, and a coolant outlet aft of the intake for discharging the coolant from the interior space. The intake resides forward of the throat. During operation, a coolant film flows along the radially inner surface of the nozzle. The projection encourages a portion of the coolant film to enter and flow through the coolant flowpath to convectively cool the nozzle. The location of the intake, forward of the throat, takes advantage of the locally high gaspath pressure to ensure that an adequate quantity of the coolant enters and flows through the coolant flowpath.